Coleman Mach 3 A/C Maintenance – Stop RV Leaks Fast

If you’ve ever walked into your RV living room and felt a surprise drip on your head, you’re not alone. Recently, our Coleman Mach 3 rooftop air conditioner started leaking water like a sieve into our living area. The culprit? Clogged drain holes — and a whole lot of sludge.

Here’s how I climbed up, dug in, and fixed the issue — and how you can too.

Table of Contents:

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The Problem: A/C Unit Leaking Inside the RV

We noticed water dripping from the A/C intake into our living room. Not just a little moisture — drips, pooling on the floor. Not great, especially in high humidity.

After ruling out rain and roof leaks, I climbed up to the roof and removed the shroud on our Coleman Mach 3. Sure enough, the drain tray was full of water, and three of the four drain holes were clogged with thick sludge.

Tools and Supplies I Used

Screwdriver or nut driver
Pipe cleaner or small wire
Shop vac or compressed air
Microfiber cloth
Frost King ACF19 Foam Coil Cleaner
Soft brush (for condenser coil)
Fin comb (optional)

Step-by-Step: Full Coleman Mach 3 A/C Cleaning

This maintenance routine handles it all in one go — cleaning the drain tray, evaporator coil, condenser coil, and checking the gasket. Here’s how I do it:

1. Shut Off Power

Before anything else, cut the power to your A/C system at the breaker or disconnect the 12V control circuit. Safety first.

2. Remove the Rooftop Shroud

Climb up safely and remove the A/C shroud using a screwdriver or nut driver. Watch for wasp nests, leaves, or other debris while you’re up there.

3. Clear the Drain Tray and Drain Holes

If you’ve previously sealed the evaporator box with aluminum HVAC tape (like I do for maximum efficiency), you’ll need to remove that tape to open it up. I retape the seams every time I do a full service. It keeps airflow tight, reduces heat intrusion, and helps the A/C work smarter, not harder. Remove the cover over the evaporator section to access the drain pan and evaporator coil. Inspect the condensate drain tray — mine was full of standing water. Locate the four corner drain holes. I found three of mine completely clogged with sludge. Use a pipe cleaner, stiff wire, or compressed air to clear them out. Wipe down the tray with a cloth and antimicrobial cleaner

Pro tip: Since you’ve already opened up the unit to access the drain tray, it’s the perfect time to clean the evaporator and condenser coils, too. It only takes a little longer, and it’ll help your A/C run more efficiently while preventing future issues.

4. Clean the Evaporator Coil

Place a towel or a couple of rags over the large hole leading into the RV (usually the air return or duct opening). This helps catch any cleaner or debris that might drip down while you’re working.
Before spraying the coil, remove the thermostat probe from the evaporator fins. It’s usually pressed into the coil to sense temperature and control the compressor cycle. Removing it prevents the probe from getting coated in foam or bent during cleaning.
Inspect the coil for dust, bugs, or road grime.
Spray Frost King ACF19 Foam Coil Cleaner directly onto the fins — be generous
Let the foam sit and expand for 5–10 minutes.
Use the built-in brush on the cap to scrub the coil in the direction of the fins gently.
Repeat the process for a second round of cleaning. This really helps remove anything stuck deep in the fins.
If you notice any bent fins, gently straighten them with a fin comb to maintain optimal airflow.
Let the cleaner drain into the now-cleared drain tray.

Pull out the thermostat probe during cleaning

5. Clean the Condenser Coil

The condenser coil is located at the rear of the unit
Brush away loose debris using a soft-bristle brush or a vacuum with a brush attachment
Spray the coil with Frost King ACF19 Foam Coil Cleaner, just like the evaporator. It foams up and breaks down dirt and grime
Let the foam sit for 5–10 minutes, then allow it to naturally drain or evaporate
If needed, lightly brush the fins again to help remove any remaining gunk

Important: Don’t use high-pressure water or hard brushes on either coil — the fins bend easily and restrict airflow if damaged

6. Check the Fan Blades and Motor

While everything is still open, take a moment to inspect the fan blades and motor
Make sure the blades are free of debris and spinning smoothly
Check that nothing is rubbing, wobbling, or looking out of balance
A clean, balanced fan reduces motor wear and improves efficiency

7. Inspect and Tighten the Gasket Bolts

The A/C is sealed to the RV roof with a foam gasket and four mounting bolts. If these are too loose or too tight, you’ll get leaks or compression damage
Check all four bolts and snug them up evenly. Don’t overtighten — you want the gasket to compress slightly but not flatten

8. Reassemble and Restore Power

Replace the evaporator cover. Re-tape the evaporator coil box seams with fresh aluminum tape to seal it back up tight. Replace the rooftop shroud
Turn your power back on
Run the A/C and check for leaks inside and out

Maintenance Tips for the Future

Clean the drain tray and coils every 3–6 months, especially in humid environments
Keep your interior air filters clean to prevent coil freeze-up
Level your RV to ensure proper water drainage
Spray antimicrobial cleaner in the drain pan to slow mold growth
Routinely check the condenser coil for dirt and obstructions

Want to Be Proactive?

If you want to go the extra mile, some RVers add these upgrades:

Install drain tubes from the tray to the RV side to redirect water
Drill slightly larger drain holes (carefully) to reduce the chance of clogs
Install vent screen kits to keep wasps and bugs out of your A/C unit

FAQ: Coleman Mach 3 A/C Maintenance

How often should I clean my RV A/C coils?
Every 3–6 months, depending on your climate and how often you run your A/C. More often, if you’re in humid or dusty areas.

Can I use household cleaners instead of coil cleaner?
No. Always use a non-rinse, A/C-safe foam coil cleaner like Frost King. Household cleaners can damage the aluminum or leave residue.

Do I really need to remove the thermostat probe?
Yes. If you leave it in, foam cleaner can coat it or knock it loose, causing erratic cooling or freeze-ups.

Can I skip sealing the evaporator box with tape?
You can — but sealing it with aluminum HVAC tape helps improve airflow efficiency and reduces hot air intrusion.

Is this safe to do myself?
Yes, if you’re comfortable climbing on your RV roof and taking your time. If not, hire a mobile tech — it’s still a lot cheaper than ceiling damage or A/C failure.

Final Thoughts

If your Coleman Mach 3 is dripping water inside, don’t freak out — and definitely don’t rush to call a mobile RV tech. This is one of those DIY maintenance tasks that just takes an hour, a ladder, and a little patience.

That said, August is absolutely not the ideal time to do this. It was brutally hot up there on the roof while I was sweating through this project. Take it from me — spring or fall is a much better time to knock this out while the temperatures are manageable. Don’t be lazy like I was and let it slide until it becomes a soggy ceiling emergency.

For full-time RVers like us, staying cool means staying proactive. Doing this cleaning regularly has saved us from a soaked ceiling and a sweaty night more than once.

Stay cool and dry out there

Recommended Gear

Here are the tools and products I used:

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